Single stage tracking actuator apparatus for optical beam information storage drive system
FIELD OF THE INVENTION 
     This invention relates to tracking actuator apparatus for an information storage drive system of the type utilizing a focused optical beam to read and/or write information on data tracks of a recording medium. More particularly it relates to single stage tracking actuator adapted both to move an optical read or read/write head rapidly along a tracking axis to locate and position the optical beam on a predetermined data track and also to maintain the beam centered on the track during a read and/or write operation of the drive system. 
     BACKGROUND OF THE INVENTION 
     Information storage systems that use an optical beam to sense certain physical conditions on the surface of a storage medium representing bits of stored data are well known to have very high density information storage capacities. In order to take useful advantage of these high storage capacities, it is desirable that these systems be provided with head positioning drives, i.e. tracking actuators, that are capable of very fast seek times to position the focused optical beam on a desired data track selected by the host computer out of many thousands of such tracks. In this seek mode of operation, the head mass must be moved rapidly through optimally determined acceleration and deceleration profiles between beginning and ending track locations that could be adjacent to each other or several thousand tracks apart. Additionally, once the desired track has been reached, the head must be precisely controlled in a tracking mode of operation during which the beam is maintained in centered registry with the track, which is typically less than a micron in width, as data is being read or written. It is an object of the present invention to provide a tracking actuator for an optical or magneto-optical system that is not only capable of providing the level of tracking mode performance required by optical drives but also provides track seek (access) time performance that is at least comparable to, and in some cases better than, existing high capacity magnetic drives such as conventionally known Winchester and Bernoulli drives. 
     Various forms of tracking actuators are known to provide reasonably fast access time and good tracking control in magnetic disk drives of the aforementioned type. In a typical arrangement, the carriage for the read/write heads is mounted with ball bearing rollers on a pair of tracks, or races, and the carriage is driven by a stepper motor or voice coil motor to translate the head linearly in a radial direction over the data tracks formed on the surface of the recording disk. This arrangement has the advantage of providing a true linear tracking axis but has the disadvantage of imposing roller bearing irregularities that are not compatible with the extremely small, precise motions that are encountered in optical drives. U.S. Pat. Re. No. 32,285 is an example of a voice coil driven actuator utilized in a roller/race carriage type of hard disk drive system that employs a pair of separate voice coil motors positioned symmetrically on opposite sides of the carriage. Although not expressly mentioned in the patent, it is typical for the rollers to be mounted with ball bearings to eliminate friction. While satisfactory for magnetic drives, ball bearings suffer from fretting corrosion when subjected to the sort of micro-fine vibratory motion encountered in optical drives. Fretting corrosion occurs when the vibratory motion drives lubricant out from between the ball and race leaving a metal-to-metal contact during the period of vibratory motion. 
     The specification of the patent suggests in the introductory portion that s single drive coil somehow wrapped around the carriage could also be employed as an alternative to the pair of coils, however no embodiment of this type is disclosed, nor is it readily apparent from the disclosure how such an embodiment would be constructed. However, taking into account the pair of coils that are disclosed, it is apparent that a substantial amount of unused wire, i.e. portions of the coil not involved in developing the required motive forces, would be required to span the carriage space between the dual magnetic circuits on either side of the carriage. This unused wire is detrimental since it represents dead weight in the moving mass that undesirably increases the power consumption needed to achieve desired operating performance. 
     Moreover, purely magnetic information storage systems generally have tracking requirements that are not as precise or stringent as those found in the case of optical and magneto-optical storage system. Because of the extremely small track-to-track spacing found on optical and magneto-optical disks, on the order of 1.6 microns, information storage systems that use optical beams to scan recorded information on such narrow data tracks conventionally employ a two stage positioning arrangement. The first stage, or seek mode of operation, may typically involve a first drive apparatus, either stepper motor or voice coil, to move the entire mass of the head to rapidly position the beam in the vicinity of the desired track in as short an access time as is possible. In the second stage, or tracking mode of operation, a separate drive apparatus, usually another voice coil moved into coarse position over the tracks by the first actuator stage, is used to control the lateral positioning of just the beam objective lens to complete the positioning of the beam and to maintain the beam centered on the data track during read and/or write operation. In general, the reason for employing a separate tracking actuator for the objective lens, is the difficulty that conventional head positioning means have in achieving the required high frequency response for proper tracking error correction, as opposed to the seek mode of operation, particularly in compact, cost sensitive applications. U.S. Pat. No. 4,607,913 discloses a linear tracking actuator of the general type disclosed in the aforementioned U.S. Pat. Re. No. 32,285 but adapted for use with an optical read/write system. In this disclosure, an axially movable beam objective lens is mounted by flexures to a carriage between a pair of sleeves riding on guide rods. The sleeves are made of tetra-flouroethylene to reduce friction. A pair of drive coils are wrapped around the sleeves and have their outer turn segments disposed in longitudinal air gaps formed by elongated magnets in parallel with the associated guide rods. The objective lens for this system is carried in a separate mount spanning the parallel guide rails and is attached to the main carriage by the elongated flexures. A separate pair of radial drive coils are provided for the objective lens to provide additional drive force in proportion to the ratio of masses of the objective mount to the main carriage so that the flexures do not have to be relied on to convey the radial driving force from the carriage to the objective mount. This complex arrangement requires, among other things, that the magnets be extended to cover two sets of actuator coils. 
     In an effort to address problems of the nature described above, an early form of erasable magneto-optical disk drive was developed at Philips Research Laboratories in Hamburg, West Germany and which was reported in the Dec. 29, 1982 issue of &#34;Electronics&#34; magazine at page 45. In this arrangement, a Faraday effect optical light source and sensor assembly was mounted on the end of a bifurcated arm extending over a magneto-optical disk from a rectangular frame suspended by a pair of parallel flexure springs for friction-free movement of the arm and head along a tracking axis over the surface of the disk. This apparatus was a single stage actuator in that both the track seek and tracking modes of operation were accomplished with a single drive. The driving force for this apparatus was provided by a voice coil motor arrangement with the voice coil mounted on the frame spaced away from the light source head and with the plane of the coil turns oriented parallel to the tracking axis. 
     Although both of this voice coil, flexure mounted design proved to be operative for its intended purpose of eliminating the problems associated with track mounted designs, it did not prove to be satisfactory for commercial application. This actuator employed a beam source and detector cantilevered out beyond the flexure mount and required a separate, relatively large focus actuator to carry the tracking actuator flexure mount which added undesirable mass to the system. As such, this drive proved unsatisfactory to meet stringent performance requirements associated with competitive high capacity drive systems, and was not suitable for compact, low cost personal computer applications. 
     It is therefore an object of the present invention to provide a tracking actuator drive system for an optical or magneto-optical disk information storage system that avoids the drawbacks and disadvantages of prior actuator drive systems. 
     It is a further object to provide a tracking actuator drive system that lends itself to compact design for use in a half height format useful in personal computers. 
     It is yet a further object to provide a tracking actuator drive system that provides rapid seek times comparable to seek times of existing high performance Bernoulli and Winchester magnetic disk drives. 
     It is still a further an object to provide a single stage tracking actuator for an optical or magneto-optical disk drive system with relatively low mass that does not require a separate tracking actuator for the objective lens to maintain the optical sensing beam centered on the data track during read/write operation. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, therefore, there is provided single stage tracking actuator apparatus for an optical beam information storage drive system adapted in a track seek mode to move an optical read/write head rapidly along a drive path which is parallel with a planar surface of a recording medium and is generally parallel with a line normal to a plurality of closely spaced parallel data tracks formed on the planar recording surface thereby to position a focused optical beam generated within the head onto a predetermined one of the recording tracks, and adapted in a tracking mode to maintain the beam centered on the predetermined track during a read and/or write operation of the system. Accordingly, the apparatus of the invention comprises head means for generating the focused optical beam, the head means having first and second end portions with the beam emanating from one of the end portions. The head means has a trio of mutually orthogonal axial center lines, the first of which is a longitudinal center line passing through the end portion of the head means. The first and second of the center lines define a first central plane parallel with the recording medium surface, the second and third of which define a second central plane normal to the recording surface. 
     According to an important feature of the invention, the head means is mounted in the disk drive system by flexure means to provide the head means with friction-free reciprocal translation along the drive path. The flexure means includes at least one pair of parallel leaf springs having the free ends thereof attached respectively to the first and second end portions substantially equidistant from the second central plane thereof. In accord with another important feature of the invention, magnetic circuit means are provided for establishing mutually isolated magnetic fields across a plurality of elongated air gaps on opposite sides of the head means intermediate the end portions thereof. The elongated dimensions of the air gaps are parallel with the first center line of the head means and symmetrically disposed on either side of the second central plane, while the cross sections of the air gaps parallel with the second central plane are symmetrically disposed on either side of the first central plane of the head means. The apparatus further comprises electromagnetic coil means having a plurality of coil turns formed coaxially about the longitudinal center line of the head means and secured to the head means centrally of the points at which the leaf springs are attached to the head means. The end turn segments of the coil are spaced away from the head means symmetrically on opposite sides of the longitudinal center line thereof and are disposed in the air gaps to provide driving motive force for the actuator. Finally, the apparatus includes means for supplying tracking actuator signals to the coil means to cause the coil and head means to translate along the drive path in both the track seek and tracking modes of operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic side elevation view of a magneto-optical disk drive system embodying tracking actuator apparatus of the present invention. 
     FIG. 2 is a bottom plan view of the drive system of FIG. 1 showing details of the tracking actuator apparatus constructed in accordance with the present invention. 
     FIG. 3 is a schematic perspective view of the read/write head of FIG. 1 useful in explaining features of the present invention. 
     FIG. 4 is an elevation view of the tracking actuator apparatus of FIG. 2 seen in cross section taken midway between the front and rear end portions of the magneto-optical read/write head. 
     FIG. 5 is a side view of a magnet useful in the construction of the tracking actuator of the invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, information storage disk drive system 10 includes a base enclosure 11 having an upper mount plate 12 which forms a mounting base for the components of the system 10. An upper enclosure 13 includes provision for receiving and holding a disk cartridge 14 which has nested therein a magneto-optical information storage disk 15. A plurality of closely spaced parallel data tracks 16 (FIG. 2) are formed in grooves on the upper planar surface of disk 15 by deposition of a recording medium comprising a thin layer of suitable thermo-magneto-optical material utilizing processes well known for this purpose. As is known in the art, grooves 16 may be concentric with the center of disk 15 or they may comprise &#34;pseudo grooves&#34;, actually a single spiral groove with the center of the spiral at the center of the disk. A drive spindle 17 is driven by a spindle motor 18 and has at its upper end a disk centering cone 19 which cooperates with a disk clamp 20 to engage and rotationally drive disk 15 during read/write operation of the system 10. An electromagnetic bias coil 21 is held in place on upper housing 13 above disk 15 and is supplied with current from a source, not shown, to generate a concentrated magnetic field which is applied to the recording medium of the data tracks 16 as part of the magneto-optical data writing process. Spindle motor 18 is mounted on a mount plate 23 and is driven axially by a load drive mechanism 22 which operates, in part, to raise spindle motor 18 into engagement with disk 15 after cartridge 14 has been inserted into upper enclosure 13 to commence read/write operation and to lower spindle motor 18 to allow removal of cartridge 14 by the user of the computer in which the disk drive system is installed. 
     Disk drive system 10 is also provided with a read/write head 25 which is moved in a controlled manner from an at-rest position, shown in solid outline, between extreme positions 25a, 25b by tracking actuator apparatus of the invention along a drive path which is parallel to the planar surface of disk 15 thereby to position a focused laser optical beam 26 generated within head 25 onto a selected one of data tracks 16 within the span of data tracks between the extreme positions 26a, 26b of beam 26. Beam 26 is focused by an objective lens 31 mounted within head 25 by means of a single axis actuator that operates to move the lens axially to maintain proper focus of beam 26 on the data tracks 16 but is fixed (non-movable) in the lateral direction. Consequently, tracking actuator apparatus of the invention operates to provide single stage lateral positioning of the beam on a data track by controlling movement of head 25 both to rapidly seek a selected data track as determined by the host computer (not shown) as well as to employ microscopically small tracking error corrections at very high frequencies or rates needed to maintain the beam on the selected data track during the read/write operation of the drive system. 
     Referring now to FIG. 2, there is shown a bottom plan view of single stage tracking actuator apparatus 30 adapted to carry out the aforedescribed operation in accordance with the invention. To this end, actuator apparatus 30 includes head means 25 comprised of a housing having end portions 25&#39; and 25&#34; and which is preferably made of a strong, light weight material, such as magnesium. Head 25 includes within the housing optical read/write components for generating the high intensity laser beam 26 (FIG. 1) focused by objective lens 31 in front end portion 25&#39; through an aperture 32 in mount plate 12 onto data tracks 16 of disk 15. A more complete description of the optical structure of head 25 may be found in copending U.S. patent application Ser. No. 234,767 filed Aug. 8, 1988. 
     To aid in defining the spatial relationships of the various components of the tracking actuator of this invention, FIG. 3 schematically illustrates head 25 with a trio of mutually orthogonal axial center lines 27,28 and 29. The first center line 27 extends longitudinally through end portions 25&#39;, 25&#34; parallel to the surface of disk 15. Thus, longitudinal center line 27, together with the second center line 28 extending through the longitudinal sides of head 25, define a first central plane which is parallel to the recording surface of disk 15. Similarly, center line 28 and vertical center line 29 define a second central plane which is perpendicular to the recording surface of disk 15 and to the radial line extending through the center of disk 15. 
     Flexure means including a pair of elongated, flat leaf springs 33, 34 are provided to hold head 25 suspended with friction-free translatable motion along the drive path between extremes 25a, 25b. To this end, the free ends of springs 33, 34 are secured to the head end portions 25&#39; and 25&#34;, respectively, by suitable means such as bolts 35. The distal fixed ends of springs 33, 34 are secured to mount plate 12 near the side thereof by bolts 36 affixed to extension arms 40a and 40b of ring core 47 which, in turn, is bolted by bolts 37 onto positioning pads 38 formed on the undersurface of mount plate 12. In this manner, head 25 is suspended, in cantilever fashion, in position under disk 15. The points at which springs 33, 34 are attached to extension arms 40a, 40b lie in a line that is parallel to the radial line extending through head 25 and the center of disk 15. Additionally, the distance between bolts 36 on extension arms 40a, 40b and bolts 35 on head 25 are equal for both springs and thus head 25 is held with its longitudinal center line 27 parallel to the radial line throughout the range of translation motion. Leaf springs 33, 34 may be formed of stainless spring steel on the order of 0.002- 0.003 inches thick. In a preferred form of the invention, each of the springs has a laminate of constrained layer damping material 39 adhesively secured to the flat surfaces of the springs to dampen resonant forces on the springs during high frequency translating movement of head 25. 
     As can be seen in FIG. 2, the flexure suspension of head 25 results in a slightly arcuate path 31a for beam 26 as head 25 is translated back and forth between extreme positions 25&#39;, 25&#34;. To minimize the radius of the arc, leaf springs 33, 34 are made as long as possible within the physical constraints of the dimensions of the disk drive system. To this end the fixed ends of the leaf springs are mounted as close to the side of mount plate 12 as is reasonably possible. In a similar vein, the span between the springs is made as wide as feasible with a view to minimizing any tendency of the head to twist or rotate about its center. Consequently it is preferred that the free ends of the springs be secured on the endmost surfaces of head 25 as shown in FIG. 2. 
     Magnetic circuit means 40 and 41, comprised of separate assemblies of an elongated magnet 44 with ring core 46 and an elongated magnet 45 with ring core 47, respectively, are provided for establishing a pair of mutually isolated magnetic fields across a pair of elongated air gaps 42, 43 on opposite sides of head 25 intermediate end portions 25&#39; and 25&#34;. The elongated dimension of each of the pair of air gaps 42, 43 extends in parallel with the longitudinal center line of head 25. Ring cores 46 and 47 are each generally in the shape of a flattened ring with elongated legs parallel to the longitudinal center line of head 25. The legs closest to head 25 are shorter than the associated outer legs and, as a consequence, the side legs of each are seen to converge from the outer to the inner legs. The longer outer legs are preferred so as to minimize undesirable fringe effects in the air gaps at the ends of the magnets. It will be seen that the magnetic circuit means 41 is nested within the span of leaf springs 33, 34 which is done to aid in achieving a compact design of the overall drive system by keeping the leaf springs and magnet circuits in the same plane. As a consequence, the configuration of the ring core 47, and in particular the lengths of the inner and outer legs, is therefore selected to allow the span of flexure springs 33, 34 to skew right and left, as seen in FIG. 2, i.e. frontward and rearward in the drive system, without interference between the flexure springs and the ring core. 
     Magnets 44 and 45 are secured by suitable means, such as an adhesive, to the inner surface of the outer legs of each core and are magnetized in a direction that is parallel with the second center line 28 passing through head 25 (FIG. 3). The magnets are spaced from the inner legs of each core thus forming air gaps 42, 43 with the magnetic field flux lines extending thereacross parallel with second center line 28. As shown in FIG. 5, the vertical outline of each of the magnets 44, 45, in the elongated direction, is somewhat in the shape of a bow tie, with upper and lower surfaces 44a, 45a and 44b, 45b, respectively, gradually increasing arcuately from the centers to the outer ends to provide concave upper and lower surfaces. This configuration for the magnets is selected to provide a more uniform net force constant across the full range of skewing of the leaf springs 33, 34. To reduce the inductance of the coil, copper sleeves 48 and 49 are formed about the shorter legs of ring cores 46 and 47, respectively, to serve as conventional shorted turns. 
     The tracking actuator also includes an electromagnetic coil 50, preferably of elongated rectangular cross section with elongated side turns 50a, 50b secured by suitable means, such as an adhesive, to the upper and lower surfaces of head 25 supported by laterally extended wings 51 above and below the head. Coil 50 is affixed to head 25 intermediate the end portions 25&#39;, 25&#34; with the axial of the coil coincident with the longitudinal center 27 of the head. The coil is preferably centered between the points at which the free ends of flexure springs 33, 34 are attached to the head and is symmetrically disposed on either side of the second central plane formed by head axial center lines 28, 29. The end turn segments 50c, 50d of coil 50 are disposed in air gaps 42, 43, respectively, with the wire of the end turns orthogonal to the flux lines across the gaps. Additionally, the assembly of head 25 with the magnetic circuit means 40, 41 is such that the coil end turn segments 50c, 50d are centrally positioned, front to rear, in the air gaps when flexure leaf spring are in the at-rest, unstressed position. As seen in FIG. 4, because of the slightly arcuate nature of the drive path for head 25, the magnet-to-pole air gap width is made sufficiently wide to accommodate the effective side-to-side motion of the coil end turns 50c, 50d as the head is translated back and forth, as shown in dotted outline in FIG. 2. Similarly, the lateral spacing between the inner legs 46a, 47a is chosen to accommodate the lateral motion of head 25 resulting from the slightly arcuate nature of the drive path. A side leg of each ring core is removable, as at junctures 52 and 53, to allow assembly of the coil 50 with the end turn segments in air gaps 42, 43. 
     Means including input terminals 55 and electrical connecting lines 56 are provided for coupling tracking actuator signals from an actuator servo signal source 57 to the drive coil 50. Because of the reciprocating movement of head 25, lines 56 are preferably formed in a flexible signal bus tape harness. As is well known in the magnetic hard disk drive art, the tracking servo signals may comprise analog signals which, in the seek mode, drive head 25 with a controlled acceleration in the first half of the seek operation followed by a controlled deceleration in the second half of the seek operation. As the head approaches the desired track, the servo signal may then switch to a position servo signal in which the position of the head over the track is controlled to find and maintain the center of the desired track. A tracking error signal detected from the reflected optical beam is used for providing actual velocity and position signals in the servo loop which generates the drive signals to input terminals 55. 
     It can be seen from the foregoing that what has been disclosed is a very compact tracking actuator for an optical read beam type of information storage drive system. The smooth friction-free translatable motion of the flexure spring suspension allows the use of single stage tracking operation that significantly reduces the complexity of the objective lens mount in the head and substantially improves the track access time performance of the actuator over conventionally known optical beam information storage drive systems. 
     While there has been described what at present is believed to be a preferred embodiment of the invention, it will be appreciated by those skilled in the art that various modifications may be made without departing from the spirit and scope of the invention. For example, although an enclosed ring core is illustrated in the enclosed embodiment, it possible to employ an open-ended C-shaped core with substantially all of the flux return path occuring through the single remaining side leg. Further, head 25 of FIG. 2 includes optical means for detecting Kerr effect rotation in the polarization of optical beam 26 reflected back from the disk surface. Head 25 might alternatively include optical means for detecting Faraday effect rotation occuring in the transmission of the beam through the the media of the disk. In this event, leaf spring 33 and front portion 25&#39; of head 25 would be bifurcated to fit over the disk 15 to allow the beam objective lens and beam sensor to fit on opposite sides of the disk. With the foregoing in mind, it should be clear that it is intended that the invention is to be limited only by the scope of the claims appended hereto.